Late 1970s SMPS Design Query

Those resistors are useless, like most of the other parts. There's no DC to be balanced between the caps.

Yes, the bulk storage ones. All the others are Philips's own - with a poor rep for reliability I gather. Not that *any* electros are much good in this respect, of course.

Yeah, it's kind of like something is making it run flat-out; someone else also said to check for shorts on the secondary side (diodes prime suspect). Unfortunately I think this is not really possible in-circuit so will require a leg lifted in each case - not good with an old delicate pcb with flaky traces.

With the board unplugged, all output rectifiers should give a non-short reading.

RL

Thanks for that. I'll check them out tomorrow as it's getting late here now and there'll be murders if I don't get off this 'puter PDQ.

Wow, thanks for all that, Dimitrij. A lot to contemplate in your post so I'll have to report back tomorrow on my findings as it's late here now.

g'night!

Thanks for confirming my suspicions on the component count, John. Tempting to start afresh, really.

It's easier to understand if you unravel it a little first. (below) (I've left out values I couldn't read.)

Clearly the thing is hard-switched by V1806. L1804 turns the hard square transformer drive into a current ramp, and I'm too lazy to check whether this really resonates with C1807-8.

I suspect it does not. There's no return for the south end of L1804 once V1804 turns 'off,' so any 'resonance' in the drive circuit is defeated. Instead we see some snubber loops (omitted) intended to kill the stored energy, and 'transformer' T1801's inductance rings with the resonating capacitor C1807-8.

So, I suspect it's mostly just a flyback current-mode switcher, turn on V1804 to charge T1801, with a 'softer touch' thanks to L1804 / C1807-8, and rings a while during flyback. YMMV.

.------. +HV T1801 |~ +|--------+---+-------------+---+--------. .---> | | C1802 +| | R1801 | | R1818 )||( |~ -|--. --- .-. C1808 === 10M )||( '------' | --- | | 30nF | | )|| '---> | | '-' +---+ )|| .----------\\ ### | | | | R1817 ._)||( 1.5kV +---+ C1807 === 10M | ||( C1803 +| | 30nF | | | ||( 220uF --- .-. +---+-------' ~~~ 350V --- | | R1802 | ~~~ | '-' | | | )|| L1804 ### ### )|| 14.4uH )|| | +Vaux BAV12 | --- .---|--| V181x | | | | |>. BC549C | '-' -----' | | | |11 ### | ### | | +---

not drawn like a Tek schematic. Maybe HP.

HP was still using a lot of heavy iron, Tek not so much. When HP did incor porate switchers, they were more likely to use discrete control and driver circuits rather than custom parts or commodity ICs made for TV sets.

SMPSes were hard for everybody to get right. The HP 8662As and 8663As were heretical for their use of switchers in the early 1980s. Most of those ge nerators have a 20-kHz spur at about 100-110 dBc, sometimes higher. Nobody seems to know how to get rid of it, then or now (I've tried.) All of the surviving ones have inverter boards that were swapped out by the service de pots at least once or twice. HP had some of the sharpest guys around -- an d you know they didn't put the 'B' team on that particular project! -- but they still had a hard time with the PSU.

I have a lot of respect for the engineering behind the cheap, reliable swit chers we have today. There's nothing less sexy than a power supply, or mor e important.

-- john, KE5FX

Okay, that makes sense, with stored energy in the 'transformer' (it's really an inductor), but I don't see that the drive is synchronized to or otherwise related to the transformer resonant frequency.

Was that common practice? I guess the two couldn't get too far out of phase in between drive pulses anyhow...

The topology's motivation must be lower noise inside the instrument, or lack of fast-recovery rectifiers :-).

It looks quite repairable--I certainly wouldn't dump it. Check the discrete semi's and passives with the power off, check the bulk electros, look for switching waveforms, and Bob's yer uncle. (Or CD's uncle, in this case.)

Cheers, James Arthur

That's what the FREQ trimpot is for.

FUNCTION

The photo site's down--is there really a FREQ trim?

That could sure screw up a lot if it were off-tweak.

Cheers, James Arthur

Yes, and a few other trimpots.

Joergian sound effects maybe.

.... Phil

Reliability of ecaps is actually just fine, but you can't expect them to last 40 years.

--sp

Opens, shorts and pn junction drops.

With the unit unplugged, check supply rail impedances.

Philips passives are pretty long-lived.

Don`t fiddle with trimpots (unless they`re obviously broken), until after the supply rails are functional again. You say it tries to run. Checking the rails while this is happening can also show up disproportionally loaded (ie lower than the normal ratio) rails and point to downstream faults.

Fooling with the puter won`t fix the scope, either.

RL

Digital scopes are small, light, run cool, and seem to be very reliable. Once you figure out aliasing, it's not a problem.

Single-shot acquisition, measurement cursors, frequency measurement, ability to save waveforms to flash sticks, signal averaging, all are wonderful.

Don't forget BSODs--analog, eat your trace out.

Maybe soon we can choose between Android and Win10!

Cheers, James